Boeing 777 G-YMMM
London Heathrow
17 Jan ’08
BRIEFING TO IASCC 9 September 2010
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Slide 1

( No 3 Jet Fuel, PRC )
( Min temps: SAT -76ºC; TAT -45ºC; Fuel -34ºC )
Slide 2
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1,600 ft agl to TD
4 sec grid spacing
Thrust
Levers
EPR actual and command
FMV (QAR)
Fuel Flow
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Slide 3

Fuel temp at
590 ft agl was
-22 deg C
Min Fuel Temp in cruise -34 deg C
PALT
TAT
FDR Data
Takeoff fuel temp was -2 deg C
Accident Flight Track from FDR
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Slide 4

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Slide 5

• No pre-existing defects found with electrical, hydraulic, autoflight, navigation systems or flying controls
• HIRF/EMI eliminated by testing – the power levels required to affect the EEC would have affected the electrical, navigation and communications system first.
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Slide 6

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Slide 7

Engine
Strut pipes
Main tank
Centre tank
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Slide 8

Engine - HP pump cavitation marks
• Fuel Pump 0019 (LH Engine)
– Built Oct ’99, Never overhauled
– A/C Boost pump debris on Impellor
– Abnormal cavitation markings on bearing thrust faces and HP housing at discharge window
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Slide 9

• Dissolved water
– Molecule of water attached to a hydrocarbon molecule. As temperature drops becomes entrained water.
• Undissolved water
– Entrained water, often referred to as suspended.
• Suspended as tiny droplets in the fuel settles to form free water.
– Free water
• Visible water that collects on bottom of tanks.
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Slide 10

• Only entrained and free water form ice.
• Ice crystals form at -1 to -3 Deg C.
– Density similar to fuel, so float in fuel.
• Critical Icing Temperature ~ -8 Deg C.
– When ice crystals start to stick to their surroundings.
• -18 Deg C
– Crystals adhere to each other and become larger.
• Below -20 Deg C little is known about the properties of the ice.
Air Accidents Slide 11
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• Fuel was of good quality.
• Fuel freezing point was -57 Deg C.
• Compared with 1,245 batches of Jet A-1 tested in UK during 2007.
– Distillation range average.
– Freezing point slightly below average.
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Slide 12

• Dissolved water, 3 ltr (40ppm).
• Undissolved water (entrained and free), maximum of 2 ltr (30 ppm).
• This Water would have been evenly spread across the fuel system at the start of the flight.
• Water also introduced through the vent system during the flight, approximately 0.14 ltr.
• Plus any water remaining from previous flights.
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Slide 13

• Beaker Test
– Small scale test in climatic chamber.
– Used simulated fuel system components.
– Establish how ice might accumulate and restrict flow.
• Fuel rig testing
– Actual components from B777.
– Establish if ice could build up in the system and restrict the flow.
– Use fuel preconditioned with water or inject water directly into boost pump inlet.
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Slide 14

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Slide 15

0C
-50C
Water ice forms (-1 to -3C, 31 to 27F )
Sticky range (-8 to -20C, 23 to -4F )
Ice starts to adhere to metal (-9C, 16F )
Ice at most stickiness (-12C, 1 0F )
Ice adheres strongly to metal surfaces (-17C, 0F )
Ice takes on a more crystalline appearance below -20C, ( -4F )
Ice lacks the properties to bridge orifices (-25C, -23F )
Spontaneous formation of ice crystals from super cooled water
(-24C, 11F )
Lowest temperature super cooled water can exist in aviation fuel
(-51C, -60F )
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Slide 16

Sight
Glass
FOHE
Spar Valve
Sight Glass
Boost Pump
Flexible Hose
LP/HP Pump
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Boeing Proprietary
Inlet Screen
Slide 17

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Slide 18

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Slide 19

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Slide 20

Fuel pipes in main tank
LP pump
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Strut pipes
Slide 21

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Slide 22

• Ice can accrete on the inside of fuel pipes and on inlet screens.
– Thickness depend on fuel temp and flow.
• It is possible to restrict the flow through the
FOHE with cold fuel and low levels of water simulating a sudden release of ice.
• Blocks of ice unlikely to have caused restriction.
• Problems with repeatability of some of the results.
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Slide 23

"the extraction of previously unknown, and potentially useful information from significant quantities of data“
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Slide 24

Facts from the accident flight
• Fuel temperature at takeoff -2 degC
• Minimum fuel temperature in the cruise -
34 degC
• Minimum TAT in the cruise -45 degC
• Fuel temperature on final approach -22 degC
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Slide 25

BA/United/Cathay ~60,000 flights
(Apr06 to Mar 08)
Minimum fuel temperature, -12 deg C and below
10,00
-45 -40
G-YMMM
-34 Fuel
-45 TAT
-35 -30 -25 -20
Fuel Temp
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-15 -10 -5
0,00
0
-10,00
TAT
-20,00
-30,00
TAT1
-40,00
-50,00
-60,00
Slide 26

The accident flight WAS NOT unique with respect to fuel temperatures experienced during takeoff, cruise or approach phases
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Slide 27

Through the complementary use of data mining and laboratory tests, efforts were focused on the activity of two parameters:
Fuel Flow and Fuel Temperature
The following slide identifies the combination of Fuel
Flow and Fuel Temperature parameters which were unique to the accident flight
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Slide 28

1. Fuel Temperature at take off below 0°C and remaining below 0°C during all phases of flight
2. Max Fuel Flow in cruise less than 10,000 pph
3. Fuel Temperature during approach less than -15°C
4. Max Fuel Flow greater than 10,000 pph during approach
5. Max Fuel Flow during descent less than 10,000 pph
ONLY MMM ACCIDENT FLIGHT MET ALL 5
CRITERIA FROM ~13,000 FLIGHTS.
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Slide 29

• Engines rolled back due to reduced fuel flow with no increase although FMV opens fully.
• No technical problem with the aircraft or its systems that could explain the event
• Only physical evidence – HP pump cavitation
• Reasons for HP pump cavitation – a restriction of the fuel flow to the pump
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Slide 30

• Ice can accrete on:
– Fuel tank surfaces
– Boost pump Inlet screen
– Pipework – both rigid and flexible
– Valves within the fuel system
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Slide 31

• The FOHE can become blocked when water is introduced to cold fuel creating a ‘snowball’
• The effect of the blockage changes at different fuel temperatures & fuel flows (above certain temperatures and below certain fuel flows, the
FOHE can successfully melt the ice)
• The system needs to be ~95% blocked to cause the reduced fuel flow
• Ice can accrue within the airframe fuel system and be released, dependent on fuel temperatures and flow rates
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Slide 32

• Fuel flow restricted due to ice formed from water that was naturally occurring in the fuel
• The ice accreted over a long period, with low fuel flows whilst the fuel temperature was in the ‘sticky’ range
• The ice was suddenly released, probably due to demands for higher fuel flow during final approach, but could be due to other factors such as increasing temperatures or turbulence
• The sudden ‘avalanche’ of ice blocked the FOHE, which was unable to melt it
• G-YMMM was always within its certificated operating envelope and there was no evidence of abnormal water quantities in the fuel
• No tests for this threat existed at the time of certification
(and will they in the future?)
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Slide 33

• Interim flight crew procedures to clear accumulated ice at a safe altitude
• Modifying the FOHE to resist this unforeseen threat (already underway by
Rolls-Royce)
• Investigating other airframe/engine combinations for vulnerability to this phenomenon
• Changing certification requirements to accommodate the new threat
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Slide 34

It is recommended that the Federal Aviation Administration and the European Aviation Safety Agency jointly conduct research into ice accumulation and subsequent release mechanisms within aircraft and engine fuel systems .
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Slide 35

Air Accidents
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Slide 36